For my understanding, it is optional to turn on journaling on physical files in AS/400. However, when you create a SQL collection in DB2/400, a journal and journal receiver will be created automatically. The journal created in the SQL collection is normally the journal used for logging all changes to SQL tables.
When a table is created into the collection, it is automatically journaled to the journal (QSQJRN).
For other platforms like AIX and W2K, the logging function in DB2 is the default feature.